1. Technical Field of the Invention
This invention relates generally to the field of ground-based satellite communications equipment. More specifically the present invention relates to lightweight, portable, ground satellite communications terminals and stowable antenna structures to be used therewith.
2. Background
Communication by satellite is essential in remote locations of the world where terrestrial communications networks do not exist. Moreover, when moving about remote locations, satellite communications equipment must be mobile. Smaller, lighter satellite communications equipment affords greater mobility. Satellite communications in the higher frequency bands such as X, K and Ku require a minimum transmit and receive directed gain that is much higher than the non-directional gain of handheld satellite transceivers in the L- band. Therefore, to achieve the necessary directional gain, mobile satellite transceivers in the X, K and Ku bands require directional antenna systems generally comprising parabolically shaped reflecting surfaces.
Generally speaking, while electronics have become smaller and more efficient over the years, minimum antenna size remains bounded by the physics of electromagnetic radiation and the need for larger physical antenna size (i.e., aperture) to achieve a higher directed gain. It is not uncommon for antenna systems to comprise the least transportable component of modern portable satellite transceivers.
Efforts have been made to achieve a higher degree of transportability of satellite communications antenna systems. Early efforts employed umbrella-like unfolding antennas comprising Mylar material stretched over lightweight metallic frameworks. Other efforts incorporated parabolic-shaped recesses into the satellite terminal enclosures themselves. Many others efforts involved assembling sections of flat or semi-flat panels into mosaics to achieve a larger reflecting surface. While some of these designs may indeed increase directed gain at low satellite frequencies such as in the L-band, they provide inherently unacceptable directive gain at X, K and Ku bands. The design constraint which prior attempts face at higher frequencies is their inability to provide true parabolic reflecting surfaces necessary for narrow, focused (i.e., directed) beamwidths required not only for gain, but also for discriminating among adjacent geostationary satellites position in equatorial orbits.
3. The Prior Art
U.S. Patent Application Publication 2005/0212715 A1 to Saunders (hereinafter, Saunders) attempts to overcome the effects of rain fade by increasing the physical reflecting surface of a fixed antenna reflector by adding extensions around its periphery. The invention in Saunders, however, provides no means for compacting the fixed portion of the antenna reflector. Therefore, the invention in Saunders would not solve portability issues in transportable satellite communications terminals.
U.S. Pat. No. 5,019,833 to Nonaka et al discloses a parabolic antenna for television signal reception that affords a degree of transportability by virtue of having its means for positioning incorporated into the rear of the parabolic antenna where both comprise a common assembly joined by hinges. The problem not solved by Nonaka is reducing the transportable size of the parabolic antenna reflector.
U.S. Pat. No. 4,862,190 to Palmer et al discloses a deployable parabolic dish antenna where alternating sections of triangular and rectangular reflector surfaces are connected about the periphery of a stationary main reflector surface by hinges. Upon deployment, the triangular and rectangular sections rotate outward to form a larger resultant parabolic reflecting surface centered about the main reflector. The problem with this approach is that the triangular and rectangular sections, when not deployed, are positioned perpendicularly to the main reflector, resulting in the overall displaced volume of the antenna structure to be as great when stowed as when deployed.
U.S. Pat. No. 3,618,101 to Emde et al discloses a collapsible parabolic antenna for use on-board satellites. The antenna in Emde employs at least one fixed semicircular segment and at least one movable semicircular segment which, when rotated into position, provide a 360 degree reflecting surface. Because this antenna is designed for automatic deployment, the movable segments remain connected to the primary axis of the antenna structure at all times. The result, therefore, is that the stowed volume of the antenna is less, but not significantly less, than the deployed volume of the antenna.
U.S. Pat. No. 5,554,999 to Gupta et al discloses a collapsible flat antenna that provides phasing so as to simulate the antenna radiation characteristics of a parabolic dish reflector antenna. Phasing is accomplished by a plurality of reactive elements responsive to different frequencies within the antenna's bandwidth. The antenna disclosed in Gupta is intended to be a flexible structure allowing stowage by collapsible folding. One limitation of this approach is that phased antennas yield optimum radiation patterns at the specific frequency their reactive elements are designed for, whereas parabolic reflecting antennas exhibit optimum radiation patterns across frequency bands. Another limitation of a flexible structure is the difficulty in physically supporting it and maintaining its orientation.
U.S. Patent Application Publication 2004/0196207 A1 to Schefter et al discloses a collapsible antenna for portable satellite terminals which employs a reflector assembly comprising a plurality of panels which may be connected to each other for deployment and disconnected for stowage. Connection of each panel to the other is by means of quarter turn quick release cam nuts. A separate boom arm is used to mount the feed assembly at a focused distance from the reflector assembly. Schefter discloses that the reflector is comprised of four (4) panels. However, the antenna design suffers from large overall size because, it is not a true parabolic structure and because it is a truncated structure, the feed focus is necessarily deep, as opposed to shallow feed focuses with non-truncated parabolic structures.
U.S. Pat. No. 5,061,945 to Hull et al discloses a lightweight, collapsible satellite communications dish antenna having a plurality of identical pre-shaped sectors joined at their apex which can be stowed by rotating all of the sectors about their apex so as to result in their lying substantially atop each other. The invention in Hull inherently requires that the sectors be made of a highly flexible material so as to be capable of being drawn into a curvature shape upon deployment while also capable of returning to a flat shape for stowage. Hull does not disclose any cognizable means for mounting a signal feed means at the dish focus.
What the prior art fails to provide and what is needed, therefore, is an antenna which (1.) is extremely compactable when stowed and (2.) still retains true parabolic reflector properties when deployed.